T-head LIGHT mailbox controller
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The T-head LIGHT mailbox controller enables two cores within the SoC to
communicate and coordinate by passing messages (e.g. data, status
and control) through the mailbox chan. It also provides the ability
for one core to signal the other processor using interrupts.

LIGHT mailbox Device Node:
=============================

Required properties:
-------------------
- compatible :	Should be "thead,light-mbox".
- reg :		Should contain the registers location and length
- interrupts :	Interrupt number. The interrupt specifier format depends
		on the interrupt controller parent.
- icu_cpu_id :  Shoule be the ICU CPU ID defined in SOC.
- #mbox-cells:  Must be 2.
			  <&phandle type channel>
			    phandle	: Label name of controller
			    channel	: Channel number
			    type	: Channel type

		This LIGHT mailbox support 4 channels and 2 types.
		channel supported:
			support 4 channels: 0,1,2,3
			Client should not request the chan ID the same as "icu_cpu_id".

		types supported:
			0 - TX & RX channel share the same channel with 7 info registers to
			    share data, and with IRQ support.
			1 - TX & RX doorbell channel. Without own info registers and no ACK support.

Optional properties:
-------------------
- clocks :	phandle to the input clock.

Examples:
--------
mbox_910t: mbox@ffffc38000 {
       compatible = "thead,light-mbox";
       reg = <0xff 0xffc38000 0x0 0x4000>,
             <0xff 0xffc44000 0x0 0x1000>,
             <0xff 0xffc4c000 0x0 0x1000>,
             <0xff 0xffc54000 0x0 0x1000>;
       reg-names = "local_base", "remote_icu0", "remote_icu1", "remote_icu2";
       interrupt-parent = <&intc>;
       interrupts = <28>;
       clocks = <&dummy_clock_apb>;
       clock-names = "ipg";
       icu_cpu_id = <0>;
       #mbox-cells = <2>;
};

mbox_910t_client1: mbox_910t_client1 {
       compatible = "thead,light-mbox-client";
       mbox-names = "902";
       mboxes = <&mbox_910t 1 0>;
};


mbox_910t_client2: mbox_910t_client2 {
        compatible = "thead,light-mbox-client";
        mbox-names = "906";
        mboxes = <&mbox_910t 2 0>;
};
